The purpose of this study is to investigate the antigenic composition of Mycoplasma pneumoniae in comparison with selected organisms in the Mycoplasmatales. Concentrated organisms will be divided into cytoplasmic and membrane fractions in order to locate the antigens which react with human antisera from persons with M. pneumoniae infections. Since the major antigen of M. pneumoniae is found in the glycolipid fraction and the glycolipids are heterogeneous, glycolipids will be purified and characterized to determine the number of serological specificities in the lipid fraction. The ultimate goal of the studies with M. pneumoniae is to determine the chemical nature of the antigens of the organisms to which humans make antibody in order to provide for efficient serological diagnosis and possibly useful vaccines. Since the organisms in the Mycoplasmatales appear markedly heterogenous serologically, a serological comparison by analytical means (Ouchterlony double immunodiffusion) will be carried out on all known species in the Mycoplasmatales (some 45 species). The purpose of the serological taxonomic study is to provide a sound basis for the choice of appropriate model systems for study of infection or the fundamental biology of the Mycoplasmatales.